What are you trying to improve exactly? At that level turn around on mobs (lag time between finishing one mob and damage on next one), possibly resists (depending on where you are and what the levels are, the long casting times can be quite betraying for a warlock, and the dot maintenance is poor when you mix in shadow bolts and immolates) and over kill (10 health left on the mob and you hit it for 500 with a 2.5 second casting spell) can be a big issue.

There isn't likely much the shadow priest lends to the mix as far as face melting, infact even maxed to the gills they don't lend much in terms of raw DPS that you notice dramatically. I find the threat to be quite a bit and the range required to mindflay upsetting at low level, usually because at that level there aren't actually tanks that can deal with it or recover ... oops i stole the warlock shadow vulnerability with a mind blast crit ... by bad ... oh your mortal strike tanking? bummer.

But I am likely bias running with variant slsl affliction locks and a coh/if priest mainly.

In the end you can track a lot of performance traits (dot maintenance and idle time would be my big items to suggest)